This invention relates to transducer apparatus and, more particularly, to a phase-sensitive transducer apparatus of the type comprising first and second relatively movable members wherein the first relatively by movable member has a plurality of windings and the second relatively movable member has a winding, and means for applying a first input signal to one of the windings of the first relatively movable member and a second input signal to another of the windings of the first relatively movable member, an output signal being developed, as by induction, on the winding of the second relatively movable member. In this type of transducer apparatus, the first and second input signals are generally sinusoidal in nature of substantially identical frequency and peak amplitude and are phase-displaced by a predetermined amount, and the output signal is substantially constant in peak amplitude and variable in phase during relative movement of the first and second relatively movable members.
Phase-sensitive transducer apparatus of the above type have been used in systems where it is desired to sence and record and/or control movement of a movable element. This is generally accomplished by kinematically coupling the second relatively movable member, above defined, to the movable element in order that they are able to move in synchronism. The first relatively movable member remains fixed and thus may be considered a stator and the second relatively movable member may be considered a rotor. The output signal from the rotor winding is phase-modulated in the sense that its peak amplitude remains constant and its phase changes during movement of the rotor relative to the stator. Then, by appropriately demodulating the output signal, a position signal may be derived which is periodic in nature in response to rotor movement wherein each new period of the signal is indicative of movement of the rotor and thus corresponding movement of the movable element.
U.S. Pat. No. 3,191,010 discloses a phase-sensitive transducer apparatus of the above-described type as used in a position measuring system. The stator of the transducer apparatus is shown with a pair of windings, which respectively receive a pair of sinusoidal signals of substantially identical peak amplitude and frequency, but phase-displaced by 90.degree.. The output signal from the single rotor winding is then a sinusoidal signal having the same peak amplitude and frequency as each of the sinusoidal inputs to the stator, but having a phase which varies as a function of the position of the rotor relative to the stator.
Phase-sensitive transducer apparatus can also be used in conjunction with a servo control system to control the direction and speed of movement of a movable element. In such context, a means would be provided for deriving a position signal from the rotor output signal which alternates during movement of the rotor and thus movable element relative to the stator between a desired peak voltage level and a reference voltage level. In accordance with known procedures, the reference voltage is typically ground, or zero voltage. In many servo control systems, velocity information is derived from the position signal, as by differentiation techniques, and both velocity information and position information are used in controlling the direction and speed of movement of a movable element. An example of one such servo system is disclosed in U.S. Pat. No. 3,839,665.
If a phase-sensitive transducer apparatus were used in conjunction with a servo control system of a type which utilizes not only the position signal as generated by the apparatus in response to movement of the movable element, but also velocity information as derived from the position signal, it is clear that great care must be taken to insure the peak amplitude of the position signal remains constant. Fluctuations and variations in such peak amplitude could adversly effect the accuracy of the velocity signal and thus the accuracy of the servo control system. Additionally, any offset of the electrical signals about ground when the actual mechanical assembly is at a "zero" location will cause the servo system to be inaccurate.
In most contemporary servo systems utilizing a position signal as derived from a transducer apparatus, whether phase or amplitude sensitive, movement of the controlled movable element is generally detected by sensing "zero-crossings" of the position signal. By the term "zero-crossings" in its broader context, it is meant those portions of the position signal which cross an imaginary line half-way between the positive-going and negative-going peaks. As alluded to above, in a phase-sensitive transducer apparatus, the position signal traditionally alternates between ground or zero voltage and a positive (or negative) peak voltage, thereby making the "zero-crossings" equal to one-half the peak voltage. Consequently, it would be necessary, if a traditional phase-sensitive transducer were used in a servo control system employing "zero-crossing" detection, to carefully adjust the detection system for one-half peak voltage detection. It should be clear that any amplitude fluctuations and offset errors that occur in the position signal would make accurate "zero-crossing" detection extremely difficult, thereby leading to possible servo errors.
It would be desirable, therefore, to provide a phase-sensitive transducer apparatus for generating a position signal which is maintained balanced about a reference potential, such as ground, in order to enable more accurate "zero-crossing" detection. It would further be desirable if, in such apparatus, the position signal is maintained at a substantially constant peak amplitude.